There are only 2 things when swimming freestyle that will give you more power and speed. Less resistance and drag and a correct catch and pull underwater.
So the 'High Elbow Catch and Pull' phrase...
When swimming freestyle we want to keep a high-elbow catch. This means keeping your elbows closer to the surface of the water during the first part of your stroke. This puts the forearms in the position to catch the water along with the hand, adding more forward propulsion and creating less resistance.
Remember... fingertips below wrist, wrist below elbow through your entire stroke.
As you extend your arms forward to begin the catch, the point of your elbow is typically down. Rotate the point of your elbow out and toward the surface. This will allow you to get the fingers pitched down and the elbow up.
Having a good feel for the water is essential if you want to progress with your swimming.